I think it was an old classmate of mine, Bill Shakespeare, who wrote,
"The fault, dear Brutus,
is not in our stars,
But in ourselves..."

...essentially because we are very, very, human.

How we go about acclimating new fish (usually gradually and NEVER, NEVER adding any disease and ammonia laden drops from the fish water in the bag into the aquarium. (Well, ok, a few drops on the fish may get into the tank, but why inoculate the tank with a whole lot of ammonia and many disease organisms which may be floating around at the shop?)

You probably have already read
Getting Acquainted: Acclimating New Fish to a New Aquarium
which is found in
New Tank/Cycling/Setting Up/Water Changing
in Immediate Help.

If not, it really should be required reading. There are better versions of that in some of the books, so it isn't my writing but the info. which you need to encounter.

Unfortunately, most of us discover that info (me too) after having introduced fish one of the wrong ways. I shudder to think of the microbes and chemicals I inflicted upon an aquarium and my favorite fish before I knew better.

Also, we don't know what your ammonia and later your nitrite readings were in that tank. Do we know what size tank that is? Four adult guppies is just about pushing past the envelop for a new 10-gallon tank, even with appropriate water conditioners. The female, as the least dominant of the quartet, could have died of ammonia poisoning.

It seems unfair to ask new aquarists to get test kits for ammonia, nitrites and nitrates. But without them, they are almost defenseless against that type of mystery death, which is where we (all too often) poison our own fish out of ignorance, :(

I am sure that have I added too many new fish to an aquarium. And I probably added some sooner than I should have. While there are some "tricks of the trade" which may make that less devastating for the new fish, when we are doing those things, we probably don't know the tricks either.